Internationalized Email Addresses, for example প্রোটন@তৌহিদুর.বাংলা is not supported by Proton Mail. When I type my email address in Proton Mail, it weirdly tells me address is not valid.
Note that popular email service providers such as Gmail supports this.
To Reproduce:
Try to send an email to the email address given above.
It’s a good question and something they ought to support.
Their official account in the Fediverse is @protonprivacy@mastodon.social. You might want to try DMing them.